It is sometimes desirable to covertly track the movement of objects. For example, it may be desirable to catch smugglers or prevent export of export-controlled components.
Some materials used as taggants are generally fluorescent-based and must be illuminated with particular wavelengths and intensities of light to be effectively detected. These materials may not be compatible with use on some substrates, since the substrate itself may fluorescence and interfere with the emission from the taggant. They most commonly produce visible signatures. This limits the ability to covertly tag and identify the objects.
Other types of tags are electronically activated, like the ones commonly found in retail stores. These types of tags are effective, but can not be easily implemented in a covert fashion, nor read out from a distance.
What is needed is a method and apparatus for covertly marking objects. Also needed is a method and apparatus for covertly tracking the movement of objects.